This type of lock is susceptible to unauthorised opening, especially by use of keys milled or filed to resemble the correct key. This lock is also susceptible to "picking" to achieve unauthorised opening. Consequently, many systems have been proposed to ensure that the lock can be operated only by insertion of the correct key.
For example, U.S. Pat. No. 4,213,316 (Tietz) uses, as well as pin tumblers, supplementary pins located in bores in the core and biassed towards the centre thereof. To operate this lock it is necessary to use a key with two opposed locking ribs of the correct depth. These contact the supplementary pins and align them to abut the rotational path of the core. If the ribs are of insufficient depth, the supplementary pins will be short of abutment with the rotational path and, after a certain degree of rotation of the core, a pin tumbler, which is in the same plane as the supplementary pins, will partially enter the supplementary pin bore and jam further rotation of the core.
In U.S. Pat. No. 4,434,636 (Evva-Werk), the lock includes, as well as the conventional tumbler pins, auxiliary tumbler elements movable at an oblique angle relative to the keyway and being engageable at one end in arresting recesses in the housing.
U.S. Pat. No 4,377,082 (DOM) also includes auxiliary tumblers actuated, in this case, by a rolling element in the key.
U.S. Pat. No. 4,608,842 (Zeiss Ikon) employs three types of auxiliary tumblers operated by a key with opposed ribs.
It will be appreciated that all these prior art locks involve the use of supplementary or auxiliary pins and therefore manufacture of these locks requires a substantial degree of engineering. Not only do these prior art locks cost a great deal more to produce, they are also susceptible to problems due to any imprecision in manufacture.
It is an object of the present invention to provide a lock which addresses many of the same problems as the prior art, but which provides solutions with relatively simple mechanisms. In particular, it is an object of this invention to provide such a lock which does not require auxiliary tumblers.
One continuing difficulty with locks of the pin tumbler type relates to key restriction systems. In many prior art arrangements, a key blank of unique profile is provided for a customer or manufacturer in order to control or prevent unauthorised key cutting for a particular group of locks. However, the duplicating of keys usually involves the removal of material from the key blank. Under present conventional systems, key cutters may be able to locate an unrestricted key blank profile similar to a restricted key blank and to mill or file the unrestricted blank until it matches the restricted profile.
It is an object of the present invention to provide an arrangement and system such that a restricted key cannot be duplicated merely by removing material from a generally available profile.
It is a further object of this invention to provide a lock system in which, in one embodiment, insertion of an incorrect key can result in jamming of the lock.